1. Field of the Invention
This invention relates to a catalyst composition and method for the oxidation of oxidizeable components of a gas-borne stream, e.g., for treatment of diesel engine exhaust, and more specifically to the treatment of such diesel exhaust to reduce the particulates content thereof.
2. Background and Related Art
As is well-known, gas-borne streams from industrial processes or engine exhausts often contain oxidizeable pollutants such as unburned fuel and vaporized or condensed oils. For example, diesel engine exhaust contains not only gaseous pollutants such as carbon monoxide ("CO") and unburned hydrocarbons ("HC"), but also soot particles which, as described in more detail below, comprise both a dry carbonaceous fraction and a hydrocarbon liquid which is sometimes referred to as a volatile organic fraction ("VOF"), which terminology will be used herein, or a soluble organic fraction. Accordingly, although sometimes loosely referred to as an "exhaust gas", the exhaust of a diesel engine is actually a heterogeneous material, comprising gaseous, liquid and solid components. The VOF may exist in diesel exhaust either as a vapor or as an aerosol (fine droplets of liquid condensate) depending on the temperature of the diesel exhaust.
Oxidation catalysts comprising a platinum group metal dispersed on a refractory metal oxide support are known for use in treating the exhaust of diesel engines in order to convert both HC and CO gaseous pollutants and particulates, i.e., soot particles, by catalyzing the oxidation of these pollutants to carbon dioxide and water. One problem faced in the treatment of diesel engine exhaust is presented by the presence of sulfur in diesel fuel. Upon combustion, sulfur forms sulfur dioxide and the oxidation catalyst catalyzes the SO.sub.2 to SO.sub.3 ("sulfates") with subsequent formation of condensible sulfur compounds, such as sulfuric acid, which condense upon, and thereby add to, the mass of particulates. The sulfates also react with activated alumina supports to form aluminum sulfates, which render activated alumina-containing catalysts inactive. In this regard, see U.S. Pat. No. 4,171,289 at column 1, line 39 et seq. Previous attempts to deal with the sulfation problem include the incorporation of large amounts of sulfate-resistant materials such as vanadium oxide into the support coating, or the use of alternative support materials such as alpha-alumina (.alpha.-alumina), silica and titania, which are sulfation-resistant materials.
Generally, the prior art has attempted to deal with these problems by dispersing a suitable oxidation catalyst metal, such as one or more platinum group metals, upon a refractory metal oxide support which is resistant to sulfation.
Co-Pending Parent Application
Co-pending parent application Ser. No. 07/798,437 filed Nov. 26, 1991 discloses and claims a catalyst composition and method for the treatment of diesel engine exhaust and co-pending related application Ser. No. 07/973,462, a continuation-in-part of Ser. No. 07/798,437, discloses and claims an oxidation catalyst and method for the oxidative purification of pollutants in gas-borne streams generally, including diesel engine exhaust streams. The catalyst composition comprises a catalytic material consisting essentially of bulk ceria and bulk alumina. Optionally, a low loading (0.1 to 15 g/ft.sup.3 of catalyst volume) of platinum or a conventional loading (0.1 to 200 g/ft.sup.3 of catalyst volume) of palladium may be incorporated into the catalytic material of the parent applications.